The ongoing need for more efficient power sources has generated strong interest in fuel cell research. As opposed to batteries, fuel cells are energy conversion devices in which electrodes are supplied with a continuous feed supply of both fuel and oxidant, resulting in the conversion into electrochemical energy. Fuel cells are efficient and have little to no emissions.
Hydrogen gas has been studied as the fuel supply for fuel cells; however the inherent safety, handling and storage problems associated therewith present significant drawbacks. As a result, alternative fuel sources such as alcohols and formic acid are being explored. The alcohol is fed directly into the cell and undergoes oxidation at the anode while oxygen is reduced at the cathode.
Among these, methanol (MeOH) has been studied in direct methanol fuel cells (DMFCs), which are useful for many portable power applications and micro power applications such as, laptop computers, cell phones, etc. As a result, DMFCs have been an area of intense research directed toward alternative sources of energy.
As a liquid, methanol can integrate effectively with many applications of DMFCs, including transmissions and distribution systems that currently exist. As a fuel, methanol is advantageous in terms of also being readily available from renewable sources from biomass such as wood. Thus, the incorporation of DMFCs as alternative energy sources in many systems would reduce reliance on more commonly used energy sources such as oil and natural gas, rendering DMFCs of considerable interest from the perspective of green technology pursuits. Methanol, while having advantageous handling and storage properties along with high energy density, presents significant challenges in application to catalytic reactions necessary for use in DMFCs. Specifically, many catalysts have insufficient activity to completely oxidize MeOH, resulting in by-products of intermediate oxidation such as aldehydes and acids.
Platinum (Pt) has long been used as the major component of anode electrocatalysts for electro-oxidation (EO) of methanol in direct MeOH fuel cells (DMFCs) (J. Appl. Electrochem., 1992, 22, 1-7). However, two major, long standing obstacles still exist that prevent large scale practical applications of the DMFC. One is the carbon monoxide (CO) poisoning during the EO of MeOH, which quickly lowers the catalytic activity of Pt. The other is the high loading of Pt needed in the anode to sustain the performance, which noticeably increases the cost of the whole fuel cell system.
Numerous efforts have been made both to improve the CO tolerance and to reduce Pt loading (Langmuir, 2003, 19, 6759-6769; Phys. Chem. Chem. Phys., 2007, 9, 5476). For both purposes, binary or ternary Pt-based metallic/metal oxide catalysts, such as PtRu (J. Phys. Chem. B, 2002, 106, 9581-9589), PtNi (J. Phys. Chem. B, 2002, 106, 1869-1877), PtSn (J. Power Sources, 2007, 166, 87-91), and PtRuTiO2 (Electrochem. Commun., 2007, 9, 563-568) have been studied, among which, the PtRu alloy has been shown to have improved practical performance (Platinum Met. Rev., 1996, 40, 150; Catal. Today, 1997, 38, 445-457).
Consequently, most of the recent research in this field has focused on manipulating PtRu from different perspectives, such as varying the molar ratio between Pt and Ru (J. Phys. Chem., 1993, 97, 12020-12029), improving synthetic methods (Appl. Catal., A, 2005, 285, 24; J. Phys. Chem. C, 2008, 112, 1479), and adopting different carbon supporting materials (Chem. Commun., 2004, 2766-2767; Electrochim. Acta, 2006, 52, 1697-1702; Int. J. Hydrogen Energy, 2008, 33, 427-433).
Recently, Brankovic et al. adopted a spontaneous deposition method (that was first used in reverse; depositing Ru on single crystal Pt surfaces, see Langmuir, 1997, 13, 5974-5978) to decorate the surface of carbon-supported RuNPs with Pt (Electrochem. Solid-State Lett., 2001, 4, A217). The method involved a necessary step of reducing RuNPs with hydrogen gas at relatively high temperature (300° C.). The resulting NPs, according to that work, offered the advantage of maintaining the activity towards CO tolerance with a much reduced Pt loading of ˜10 wt % compared to commercially available E-TEK PtRu (1:1) which has a Pt loading of ˜66 wt %. More recently, Kuk and Wieckowski also applied a similar method to cover Ru and carbon-supported RuNPs with different Pt loading using repetitive hydrogen reduction and spontaneous depositions (J. Power Sources, 2005, 141, 1-7). While the Pt packing densities (PDs) were determined using inductively-coupled plasma mass spectrometry (ICP-MS), the analysis of the true surface coverage and the associated activity was complicated by the possibility of Pt penetrating into RuNPs and the observed sintering effect due to high temperature reduction.
Although this spontaneous deposition method opens up a promising way of fabricating anode materials of low Pt loading, handling high temperature hydrogen reduction is technically less appealing, and repetitive hydrogen reduction and spontaneous depositions are often tedious and struggle to achieve quantitative control of the Pt coverage. Furthermore, the procedure would be difficult to implement on a large industrial scale.
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